An immune system is a system by which a body protects itself from attacks by foreign viruses and bacteria, as well as cancer cells. Although many types of cells are involved in the immune system, the important roles the macrophage plays in the system is recognized. Macrophage is ubiquitously found in all animals, and is involved in all steps of immune response, including the primary step.
It is known that various types of diseases, such as cancer, infections, and allergies, are triggered when the immune system is compromised. Conversely, if the immune system is stimulated, there is great hope for various positive effects such as oncogenic suppression, anticancer effect, anti-infection, and anti-allergy, as well as recovery of biorhythms and maintenance of homeostasis.
Thus, many researchers provide various compounds and microbes that have immunopotentiating effects, in order to improve consumers' health condition. Examples of the compounds and microbes include those containing lactoferrin hydrolysate (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 05-178759), sugars such as chitin (see, for example, JP-A No. 06-271470), trehalose (see, for example, JP-A No. 2003-81839), fucoidan (see, for example, JP-A No. 2001-181303), plant derivatives (see, for example, JP-A No. 06-56685), peptides (see, for example, Japanese Patent (JP-B) No. 2873434), interleukins (see, for example, JP-A No. 2002-3395), nucleic acids (see, for example, JP-B No. 2529605), glutathione (see, for example, JP-A No. 11-49696), amino acids (see, for example, JP-A No. 2000-281571), polylysin (see, for example, JP-A No. 2003-128589), and microbes such as enteric bacteria (see, for example, JP-A No. 06-56678), lactic acid bacteria (see, for example, JP-A No. 07-228536) and enterococcus bacteria (see, for example, JP-A No. 11-92389).
Also recently, the immunopotentiating effects of β-glucan have attracted much attention, and some have suggested methods of immunopotentiation using β-glucan which is purified from yeasts or mushrooms (see, for example, JP-A Nos. 2001-354570 and 2003-183176).
However, in order to potentiate consumers' immunity using these immunopotentiating compounds, a large amount of these compounds must be purified and given to the consumers, which involves many manufacturing processes as well as much cost and labor. In addition, properties of some of these immunopotentiating microbes are unknown and not suitable for consumption. Some of these immunopotentiating microbes must be added to food separately, if they cannot be used directly for the manufacturing processes (e.g., fermentation).
Some Patent Literatures disclose yeasts having an immunopotentiating capability, which are obtained by knocking out one or more genes selected from the group consisting of MCD4, GAS1 and CWH41 belonging to the genes on the yeast chromosome (see, for example, JP-A No. 2006-75039). The yeast disclosed in the JP-A No. 2006-75039 is able to exert an immunopotentiating effect, even when the yeast is given as whole cell and without purifying β-glucan from it as described in JP-A Nos. 2001-354570 and 2003-183176.
However, in the yeast disclosed in JP-A No. 2006-75039, the specific genes mentioned above are knocked out by recombinant DNA technologies. In recent years, with the development of recombinant DNA technologies, numerous genetically modified foods have been approved and commercialized after their safety was confirmed. The anxieties over the safety of such genetically modified foods are still considerable among consumers. Therefore, using the yeast disclosed in JP-A No. 2006-75039 in foods is commercially difficult at this moment.
Furthermore, the yeast disclosed in JP-A No. 2006-75039 exhibits a very low growth rate in media with a low osmotic pressure, as it might be expected from the fact that the yeast is lack of the genes involved in the structure of its cell wall. Thus, the incubation of the yeast poses problems such as high cost. Specifically, in order not to put pressure on the structurally fragile cell wall, sugar or other ingredients must be added to its growth media to increase the osmotic pressure so that the media has the same osmotic pressure as the intracellular liquid. In addition, if the yeast is used for fermentation of foods such as bread, there are other problems such as constricted fermentation conditions or changed food flavor due to the sugar added to make an isotonic fermentation solution.
On the other hand, mutagenesis induces mutation in DNA using mutagens, such as mutagenic compounds, UV irradiation, or radiation. And, mutagenesis induces mutation at a rate higher than that found in nature. The mutagenesis does not introduce any foreign DNA sequences (i.e., DNA sequences other than the sequences an organism naturally has), in contrast to the above described recombinant DNA methods. Thus, consumers accept foods whose genetic properties have been changed through mutagenesis as something relatively safe.
As described above, there have been no microbe such as a yeast that is highly safe, able to exert an immunopotentiating effect when taken as whole cell, easily obtained and inexpensive, usable for manufacturing of food without any particular pre-treatments, and amenable to incubation under low osmotic pressures. Therefore, keen demand has arisen for such microbes.